I haven't been able to find a precise definition in either the CAA or the FAA's materials. However, as I understand it, traffic that's "maneuvering" is traffic that's not going anywhere in particular.
If someone's flying in order to get somewhere, generally, they're going to be at a specific altitude flying in a specific direction. Course changes will be rare, and generally only occur at specific navigation points, such as VORs or VFR Reporting Points. Such traffic is easy to predict, so ATC can give you a specific heading and altitude for it, and you can assume that that information will be valid for a while at least.
On the other hand, someone that's flying for the sake of flying (e.g. someone doing aerial sightseeing, a student pilot building time, someone checking out a new aircraft, etc.) is going to be a lot harder to predict. They may turn, climb, or descend with little to no warning. That's what I've always understood "maneuvering" to mean: a warning that this traffic is unpredictable, so stay away if at all possible.